1. Field of the Invention
The present invention relates to a screw-on structure including a plastic frame, particularly to a screw-on structure that fastens a frame made up of plastic such as a cellular phone.
2. Description of the Related Art
The frames of mobile equipment such as the cellular phone are made up of plastic having lightweight and superior moldability in many cases. Many of such frames consist of two partial frames and are formed by making the partial frames screwed on with each other by a screw.
Description will be made for a conventional screw-on structure with reference to FIG. 1, FIG. 2A and FIG. 2B. FIG. 1 is a cross-sectional view showing the conventional screw-on structure, FIG. 2A is a plan view showing a plastic frame of the conventional screw-on structure, and FIG. 2B is a cross-sectional view of FIG. 2A. Note that grooves of a screw and a screw hole are not shown in FIG. 1.
As shown in FIG. 1, FIG. 2A and FIG. 2B, the conventional screw-on structure includes a plastic frame 11 provided with a through hole 15, another frame 2 where a screw hole 4 is formed, and a screw 3 inserted into the through hole 15 to be screwed down the screw hole 4. For example, the plastic frame 11 is a part of the frame that composes a non-display side of the cellular phone and the frame 2 is a part of the frame that composes a display side thereof. Then, the plastic frame 11 and the frame 2 are fastened with each other and fixed by the screw 3. Accordingly, the frame of the cellular phone is formed.
Further, the shape of the through hole 15 is circular. The inner diameter of the through hole 15 is larger than the outer diameter d of a shaft section 7 of the screw 3, and a clearance is thus provided between the plastic frame 11 and the shaft section 7 of the screw 3. The clearance can absorb a slippage between positions of the through hole 15 and the screw hole 4 when screwing on the plastic frame 11 and the frame 2 by the screw. Furthermore, the clearance can absorb an inner diameter error of the through hole 15 and an outer diameter error of the shaft section 7 of the screw 3.
However, there exist problems shown below in the conventional screw-on structure. The foregoing conventional screw-on structure has a problem that the screw comes loose when external force such as vibration and impact is applied to the screw-on structure, which is fastened each other by the screw 3. This problem is conspicuous with the mobile equipment such as the cellular phone because vibration and impact are constantly applied to the equipment. For this reason, the screw must be fixed by adding a part such as a spring washer or a method such as coating adhesive agent.
However, there exists a problem of increasing a manufacturing cost of the screw-on structure when the part such as the spring washer is added and the adhesive agent is coated. There also exists a problem that an external appearance is impaired due to existence of the part such as the spring washer or coating of the adhesive agent when the screw-on structure is applied to the mobile equipment such as the cellular phone. Furthermore, there exists a problem that it is difficult to remove a once fastened screw without hurting the plastic frame when the adhesive agent is coated on the screw-on structure.
The object of the present invention is to provide a screw-on structure where the screw is prevented from coming loose without fastening the screw by adding the part such as the spring washer and coating the adhesive agent in the screw-on structure including the frame made of plastic.
The screw-on structure according to the present invention comprises: the plastic frame having the through hole; the frame where the screw hole is formed; the screw that is inserted into the through hole to be screwed down the screw hole and that fastens the plastic frame and the frame to each other; and ribs made of plastic provided on the inner surface of the through hole and protrude toward the center of the through hole to contact the shaft section of the screw.
The inventors enthusiastically performed experiments and research in order to find out a cause of occurring loosening of the screw in the foregoing conventional screw-on structure. As a result, the inventors obtained the following findings. The inner diameter of the through hole 15 of the conventional plastic frame 11 shown in FIG. 1 is larger than the outer diameter d of the shaft section 7 of the screw 3, and the clearance is provided between the plastic frame 11 and the screw 3. Accordingly, the plastic frame 11 moves relatively against the screw 3 and the frame 2 when the external force such as vibration and impact is applied to the screw-on structure. As a result, the plastic frame 11 is wasted to be thin due to abrasion between the screw 3 and the frame 2. Thus, the screw 3 cannot maintain a necessary fastening torque, and the screw 3 comes loose.
In the present invention, providing the rib, which protrudes toward the center of the through hole to contact the shaft section of the screw, on the inner surface of the through hole of the plastic frame can prevent the plastic frame from moving relatively against the screw and the frame when the external force such as vibration and impact is applied to the screw-on structure. With this configuration, the fastening torque of the screw is maintained and the screw is prevented from coming loose. Therefore, there is no need to fix the screw by adding the part such as the spring washer or by coating the adhesive agent. Furthermore, since the rib is made up of plastic, the slippage between the positions of the through hole and the screw hole can be absorbed by distortion of the rib.
In addition, the shape of the far end of the rib may be a shape along a virtual cylinder surface whose diameter substantially equals the outer diameter of the shaft section of the screw and whose center axis substantially matches the center axis of the shaft section of the screw. Accordingly, an area where the rib contacts the screw becomes larger and movement of the plastic frame to the screw and the frame can be efficiently controlled. Even if the outer diameter of the screw or the shape of the rib has an error and the outer diameter of the screw is larger than the outer diameter of the virtual cylinder, the rib can absorb the error by deforming because the rib is made up of plastic.
Moreover, three or more ribs may be provided, the shape of the ribs in a cross-section perpendicular to the center axis of the shaft section of the screw may form an arc, and the far end of each of the ribs may contact the virtual circle whose diameter substantially equals the outer diameter of the shaft section of the screw and whose center axis substantially matches the center axis of the shaft section of the screw. The rib can be easily formed with this configuration. Further, the area where the ribs contact the screw becomes smaller, which makes it easier to absorb the slippage between the positions of the through hole and the screw and the error of the outer diameter of the shaft section of the screw or the shape of the rib.